A variety of medical devices have been used or proposed for use to deliver a therapy to and/or monitor a physiological condition of patients. As examples, such medical devices may deliver therapy and/or monitor conditions associated with the heart, muscle, nerve, brain, stomach or other organs or tissue. Medical devices that deliver therapy include medical devices that deliver one or both of electrical stimulation or a therapeutic agent to the patient. Some medical devices are implantable medical devices (IMDs) that are implanted within the patient.
Some medical devices have been used or proposed for use to monitor heart failure or to detect heart failure events. Typically, such medical devices have been implantable and, in many cases, have been cardiac pacemakers, cardioverters and/or defibrillators with added heart failure monitoring functionality. In some cases, such medical devices have monitored heart failure by monitoring intrathoracic impedance, which may provide a good indication of the level of edema in patients. While edema is a sign of many other conditions it is also a sign of worsening heart failure. Worsening heart failure may result in cardiac chamber dilation, increased pulmonary blood volume, and fluid retention in the lungs—all of which contribute to a decrease in intrathoracic impedance.
Generally, the first indication that a physician would have of the occurrence of edema in a patient is not until it becomes a physical manifestation with swelling or breathing difficulties so overwhelming as to be noticed by the patient who then proceeds to be examined by a physician. This is undesirable since hospitalization at such a time would likely be required for a cardiac heart failure patient. Accordingly, medical devices have been used to monitor impedance in patients and provide an alert to the patient to seek medical treatment prior to the onset of edema that requires hospitalization.